1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly to an apparatus and a method for performing a downlink power control in a mobile communication system employing an Orthogonal Frequency Division Multiplexing (OFDM) scheme.
2. Description of the Related Art
The OFDM scheme, which transmits data using multiple carriers, is a special type of a Multiple Carrier Modulation (MCM) scheme in which a serial symbol sequence is converted into parallel symbol sequences and the parallel symbol sequences are modulated with a plurality of mutually orthogonal subcarriers (or subcarrier channels) before being transmitted.
The OFDM scheme, similar to an existing Frequency Division Multiplexing (FDM) scheme, boasts of an optimum transmission efficiency during high-speed data transmission because the OFDM scheme transmits data on subcarriers, while maintaining orthogonality among them. The optimum transmission efficiency is further attributed to efficient frequency use and robustness against multipath fading in the OFDM scheme.
More specifically, overlapping frequency spectrums lead to efficient frequency use and robustness against frequency selective fading and multipath fading. The OFDM scheme reduces the effects of the inter-symbol interference (ISI) by use of guard intervals and enables the design and use of a simple equalizer hardware structure. Furthermore, because the OFDM scheme is robust against impulse noise, it is increasingly popular in communication systems.
A structure of a conventional communication system employing an OFDM scheme will be described with reference to FIG. 1.
FIG. 1 is a block diagram of a transmitter and a receiver of a conventional OFDM mobile communication system. The OFDM mobile communication system includes a transmitter 100 and a receiver 150.
The transmitter 100 includes an encoder 104, a symbol mapper 106, a serial-to-parallel converter 108, an Inverse Fast Fourier Transform (IFFT) unit 110, a parallel-to-serial converter 112, a guard interval inserter 114, a digital-to-analog converter 116, and a radio frequency (RF) processor 118.
In the transmitter 100, information data bits 102 including user data bits and control data bits are input to the encoder 104. Upon receiving the signal of the information data bits, the encoder 104 encodes the signal through a predetermined coding scheme and sends the coded signal to the symbol mapper 106. During the coding, the encoder 104 may employ a convolution coding scheme or a turbo coding scheme having a predetermined coding rate. The symbol mapper 106 modulates the coded bits from the encoder 104 into modulated symbols according to a predetermined modulation scheme and sends the modulated symbols to the serial-to-parallel converter 108. The predetermined modulation scheme includes a BPSK (binary phase shift keying) scheme, a QPSK (quadrature phase shift keying) scheme, a 16QAM (quadrature amplitude modulation) scheme, or a 64 QAM (quadrature amplitude modulation) scheme.
Upon receiving the serial modulated symbols from the symbol mapper 106, the serial-to-parallel converter 108 converts the serial modulated symbols into parallel symbols and sends the parallel symbols to the IFFT unit 110. Upon receiving the signals from the serial-to-parallel converter 108, the IFFT unit 110 performs N-point IFFT on the signals and sends the signals to the parallel-to-serial converter 112.
Upon receiving the signals from the IFFT unit 110, the parallel-to-serial converter 112 converts the signals into serial signals and sends the serial signals to the guard interval inserter 114. The guard interval inserter 114, which has received the serial signals from the parallel-to-serial converter 112, inserts guard intervals into the serial signals and sends the signals to the digital-to-analog converter 118. The guard interval is inserted to remove interference between a previous OFDM symbol transmitted at a previous OFDM symbol time and a current OFDM symbol to be transmitted at a current OFDM symbol time in an OFDM communication system.
Generally, null data is inserted into the guard interval. In this case, however, when a receiver incorrectly estimates a start point of an OFDM symbol, interference occurs between the subcarriers, causing an increase in an incorrect estimation rate for the received OFDM symbol. To more accurate the start point a cyclic prefix method or a cyclic postfix method is used. In the cyclic prefix method, a predetermined number of end samples of an OFDM symbol in a time domain are copied and inserted into a valid OFDM symbol. In the cyclic postfix method, a predetermined number of beginning samples of an OFDM symbol in a time domain are copied and inserted into a valid OFDM symbol.
Upon receiving the signals from the guard interval inserter 114, the digital-to-analog converter 116 converts the signals into analog signals and sends the analog signals to the RF processor 118. The RF processor 118 includes a filter and a front end unit. The RF processor 118 transmits the signals output from the digital-to-analog converter 116 over the air through a transmission antenna after RF-processing the signals.
A structure of the receiver 150 will now be described. The structure of the receiver 150 is generally a reverse of the structure of the transmitter 100.
The receiver 150 includes an RF processor 152, an analog-to-digital converter 154, a guard interval remover 156, a serial-to-parallel converter 158, an Fast Fourier Transform (FFT) unit 160, a channel estimator 162, an equalizer 164, a parallel-to-serial converter 166, a symbol demapper 168, and a decoder 170.
In the receiver 150, the signal transmitted from the transmitter 100, together with the noise that is added to the signal while the signal passes through a multipath channel, is received via a reception antenna. The signal received through the reception antenna is input into the RF processor 152. The RF processor 152 down-converts the signal received through the reception signal into a signal having an intermediate frequency band and sends the down-converted signal to the analog-to-digital converter 154. The analog-to-digital converter 154 converts the analog signal from the RF processor 152 into a digital signal and sends the digital to the guard interval remover 156.
Upon receiving the digital signal from the analog-to-digital converter 154, the guard interval remover 156 removes the guard interval from the digital signal and sends the serial signal to the serial-to-parallel converter 158. The serial-to-parallel converter 158 converts the serial signal into parallel signals and sends the parallel signals to the FFT unit 160. The FFT unit 160 performs an N-point FFT on the parallel signals and sends the FFT-processed signals to the equalizer 164 and the channel estimator 162. The equalizer 164 receives the signals from the FFT unit 160, performs channel equalization on the signals by means of channel information estimated by the channel estimator 162, and then sends the channel-equalized signals to the parallel-to-serial converter 166. The parallel-to-serial converter 166 converts the parallel signals received from the equalizer 164 into a serial signal and sends the serial signal to the symbol demapper 168.
In the meantime, the signal output from the FFT unit 160 is also input into the channel estimator 162. Then, the channel estimator 162 detects pilot symbols or preamble symbols from the signal input from the FFT unit 160, performs channel estimation by means of the pilot symbols or the preamble symbols, and then sends the channel estimation result to the equalizer 164. In addition, the receiver 150 generates a channel quality indicator (CQI) corresponding to the channel estimation result of the channel estimator 162 and sends the generated CQI to the transmitter 100 through a CQI transmitter (not shown).
The symbol demapper 168 demodulates the signal output from the parallel-to-serial converter 166 through a predetermined demodulation scheme and sends the demodulated signal to the decoder 170. Upon receiving the demodulated signal from the symbol demapper 168, the decoder 170 decodes the demodulated signal through a predetermined decoding scheme, and then, outputs information data bits 172. The demodulation and decoding schemes employed in the receiver 150 correspond to the modulation and coding schemes employed in the transmitter 100.
A mobile communication system employing the OFDM scheme requires the feedback of the CQI information from a receiver for downlink power control. For the downlink power control, a transmitter (i.e., a base station) may transmit data known to both parties to a receiver (i.e., a subscriber station), thereby facilitating the channel estimation. That is, for the channel estimation process, a signal known to both the transmitter and the receiver (for example, preamble signal or pilot signal) is transmitted.
The signal for the channel estimation may be either a preamble signal including all of the subcarriers in one symbol period or a pilot signal including at least one subcarrier in one symbol period for relatively high power transmission.
The CQI information feedback process for power control by using a pilot signal in a typical OFDM communication system will be described.
FIG. 2 is a graph illustrating positions at which pilot signals are transmitted in a frequency domain of a typical OFDM communication system. Referring to FIG. 2, in the OFDM communication system, one OFDM symbol includes a plurality of subcarriers, each of which carries a pilot signal 201 or a data signal 203. In contrast, in the case of the preamble signal described above, all subcarriers included in one OFDM symbol carry pilot symbols, respectively. The following description will be based on an assumption that the CQI information is acquired from pilot signals carried by some of the subcarriers in each symbol. However, the fedback CQI information can be obtained in the same manner from the preamble signal using all of the subcarriers.
Here, it is natural that the number of the subcarriers constituting each OFDM symbol may be optionally determined according to conditions of the system.
As shown in FIG. 2, the pilot signals are carried by subcarriers at predetermined positions from among the subcarriers of the OFDM symbol. For example, in the FIG. 2, the subcarriers colored in black carry the pilot signals and the subcarriers colored in white carry the data signals. The subcarriers for carrying the pilots signals will be referred to as ‘pilot subcarriers’ and the subcarriers for carrying the data signals will be referred to as ‘data subcarriers’.
A conventional OFDM communication system is a fixed wireless communication system in which subscriber stations or receivers are fixed at particular positions. In the fixed OFDM wireless communication system, the subscriber stations determines the CQI for each of all subcarriers received from the base station or the transmitter and feedbacks the CQI to the base station. Here, the CQI may be a Signal to Noise Ratio (SNR) or a Signal to Interference and Noise Ratio (SINR). The following description will be given on an assumption that the CQI is the SINR.
As shown in FIG. 2, the pilot signals are carried only by the pilot subcarriers at predetermined positions in a typical OFDM symbol. Therefore, the receivers must know in advance both the predetermined positions of the pilot subcarriers transmitted from the transmitter and the pilot signals carried by the pilot subcarriers. Here, the pilot signals have a predetermined sequence, and the pilot sequence is defined as a sequence for the pilot signals in a protocol agreed to between the transmitter and the receiver.
The receiver calculates channel gains in corresponding subcarriers from the received signals carried by the pilot subcarriers and interpolates the channel gain in each of the pilot subcarriers, thereby obtaining estimation values for the channel gains of the data subcarriers other than the pilot subcarriers. Further, the receiver obtains SINRs of the pilot subcarriers by dividing the estimation values for the channel gains of the data subcarriers by the noise energy. The SINRs of all of the subcarriers obtained in the way as described above, that is, CQIs are feedback to the base station. Then, the base station controls the transmission power for the corresponding subcarriers by using the CQIs feedback from the subscriber station.
The above description is based on a presumption that the subcarriers once assigned to the subscriber stations remain unchanged by channel conditions, because a typical OFDM system is a fixed wireless communication system. Meanwhile, the currently developing 4G mobile communication system is now evolving into a cellular system employing an Orthogonal Frequency Division Multiple Access (OFDMA) scheme instead of the OFDM/TDMA (Time Division Multiple Access) scheme. In an OFDMA system, a particular set of subcarriers or subchannels assigned to the subscriber stations may use different subchannels in a next transmission time interval, in order to equalize the interference between the adjacent cells for the operation in a multi-cell cellular system.
Further, the presumption that the subcarriers remain unchanged due to the channel conditions is an incorrect model for a wireless communication system reflecting the mobility of the subscriber stations. That is, it is reasonable that the subcarriers assigned to mobile subscriber stations have continuously changing channel conditions and are subjected to continuously changing interference by an adjacent cell. Therefore, the mobile subscriber stations can normally use the existing power control method only when the changing CQI is feedback for each of the subcarriers whenever it changes. However, the frequent feedback of the CQI for all of the subcarriers may cause signaling overhead and the signaling required to feedback the CQI for the subcarrers may serve as uplink interference.
In order reduce such overhead, a method of feedbacking only an average SINR for each subchannel was proposed. However, the of feedback of only the average SINR cannot actively proceed against frequency selective fading due to a multi-path fading channel and sometimes cannot avoid passively using a low channel coding rate to reduce data error generation. The use of a low channel coding rate inevitably decreases the transmission speed. Therefore, there is a need for a method for efficiently controlling the transmission power in a mobile communication system employing the OFDMA scheme.
Hereinafter, a process of controlling the downlink power by feedbacking a CQI for each of the subcarriers will be described in detail with reference to FIG. 3.
FIG. 3 illustrates a process of downlink power control in a conventional mobile communication system.
Referring to FIG. 3, a base station (BS) transmits to a subscriber station (SS) training signals known to both the base station and the subscriber station as described above, for example, preamble signals or pilot signals (step 301). The subscriber station having received the pilot signals measures the SINRs from the pilot signals (step 303). The subscriber station performs the measurement of the SINRs only for the subcarriers carrying the pilot signals instead of performing the measurement for all of the subcarriers. Further, the subscriber station estimates the SINRs for the data subcarriers not carrying the pilot signals by interpolating the measured SINRs for the pilot subcarriers.
The SINRs measured by the subscriber station, which is the CQI information, are feedback to the base station (step 305), and the transmission power for each of the subcarriers is revised based on the feedback CQI information for each of the subcarriers (step 307). As a result, the base station can transmit the data signals at the revised transmission power to the subscriber station (step 309).
In the OFDMA system, data is carried by each of the subcarriers assigned to subscriber stations, which is comprised of multiple subcarriers. Therefore, the power control as described above requires feedback of the channel quality information or the CQI information for each of the subscriber stations.
FIGS. 4A and 4B are graphs showing SINRs of subcarriers in corresponding subchannels of different subscriber stations in the same time period.
FIG. 4A is a graph showing the SINRs of six subcarriers included in a subchannel A assigned to subscriber station A and FIG. 4B is a graph showing the SINRs of six subcarriers included in a subchannel B assigned to subscriber station B.
FIGS. 4A and 4B illustrate that the subcarriers of the subchannels A and B have SINRs all of which are different from each other. Therefore, a considerably large quantity of CQI information is necessary to conduct the power control for each of the subcarrriers assigned to each of the subscriber stations. It is usual that feedback of an average SINR of the subcarriers in each subchannel is used for the power control. For example, in the case shown in FIGS. 4A and 4B, an average SINR for the six subcarriers is feedback as CQI information of the corresponding subchannel in order to conduct the power control.
As noted from FIGS. 4A and 4B, the third subcarrier in the subscriber station A (i.e. subchannel A) and the fourth and fifth subcarriers in the subscriber station B (i.e. subchannel B) have SINRs much lower than the SINRs of the other subcarriers. The use of the average value between the subchannels may be very inefficient and incorrect.
Further, if the power control is based on the third subcarrier in the subchannel A having a relatively much lower SINR, that is, if all of the subcarriers in the same subchannel A are transmitted with the same power as that for the third subcarrier in the subchannel A, it may be impossible to normally decode a signal transmitted to a subcarrier in a bad channel condition.